1. Field of the Invention
The present invention relates to an anti-skid control system for controlling braking forces of the wheels so as to achieve a short braking distance in a high braking stability in a vehicle of the type in which the wheels are driven by a power unit through torque transmission member, which receives a torsion torque and mechanically includes wheel drive shafts.
2. Discussion of the Prior Art
In the anti-skid control system, the wheel speeds are controlled by repeating pressure control cycles each consisting of increase, decrease, and hold of a brake hydraulic pressure.
Referring to FIG. 21, there is illustrated graphs showing the operation of a conventional anti-skid control system, disclosed in Postexamined Japanese Patent Publication (Kokoku) Hei-4-29580, for example. In FIG. 21, variations of vehicle speed Vb, wheel speed Vw, wheel acceleration Gw, and brake hydraulic pressure P in the anti-skid control are represented. In FIG. 21, a braking operation starts and the brake hydraulic pressure P increases, and the wheel speed Vw starts to decrease. After the wheel acceleration Gw reaches a preset acceleration .alpha.1 (negative, or deceleration), the wheel speed Vw decreases to be .lambda.1 lower than the vehicle body speed Vb at time t31. At the same time, the brake hydraulic pressure P is changed to decrease. With decrease of the brake hydraulic pressure P, the wheel acceleration Gw increases again to reach a preset acceleration .alpha.2 at time t32. At this time, the brake hydraulic pressure P is kept at a preset level. The wheel speed vw approaches to the vehicle speed Vb. At time t33, the wheel acceleration Gw decreases to below a preset acceleration .alpha.3. In turn, the brake hydraulic pressure P is changed to increase. Subsequently, this sequence of brake control operations, or the pressure control cycle, which consists of pressure increase, pressure decrease, and pressure hold, is repeated.
The anti-skid control system measures the frequency in a variation of the wheel speed Vw. If it exceeds a predetermined value of frequency, the anti-skid control system judges the present road as a rough road, and suppress the decrease of the brake hydraulic pressure P and holds or somewhat increases the brake hydraulic pressure P.
Thus, the anti-skid control system carries out the anti-skid control on the basis of the wheel speed Vw and the wheel acceleration Gw. In a case where the braking force is abruptly applied, for example, in a panic braking, the wheels that are coupled with the engine through the wheel drive shafts are abruptly decelerated. At this time, the engine is more gently decelerated than the wheels because the inertial of the engine is large. As a result, great torsion is generated in the wheel drive shafts which couple the engine with the wheels. When the wheel drive shafts receive the torsion, the wheels are repeatedly accelerated and decelerated, viz., vibrated, independently of the braking force and the road reaction.
Thus, the conventional anti-skid control system decreases the brake hydraulic .pressure independently of the road conditions because the generated torsion causes the wheels to vibrate. Accordingly, the braking distance is inevitably long. When a vehicle running on a low .mu. load surface experiences this wheel vibration., the anti-skid control system mistakenly Judges the low .mu. load surface as a bad road surface. The anti-skid control system possibly holds a larger braking force than the road reaction force. The locking of the wheels progresses, and vehicle stability deteriorates.
Thus, the conventional anti-skid control system frequently fails to optimize the braking force for the current running and road conditions.